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Journal of Structural Geology xxx (2011) 1e9

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Journal of Structural Geology

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The interaction between volcanoes and strike-slip, transtensional andtranspressional fault zones: Analogue models and natural examples

Lucie Mathieu a,b,*,1, Benjamin van Wyk de Vries b, Martin Pilato b, Valentin R. Troll c

aDepartment of Geology, Museum Building, Trinity College Dublin, Irelandb Laboratoire de Magmas et Volcans, Blaise-Pascal University, Clermont-Ferrand, FrancecDepartment of Earth Sciences, CEMPEG, Uppsala University, Sweden

a r t i c l e i n f o

Article history:Received 17 June 2010Received in revised form28 February 2011Accepted 2 March 2011Available online xxx

Keywords:Strike-slip faultsAnalogue modelsSpreadingVolcanoGuadeloupeMaderas

* Corresponding author. Department of GeologyCollege Dublin, Ireland.

E-mail address: [email protected] (L. Mathieu).1 Permanent address: 12 allée du chevalier de Lo

Braye, France. Tel.: þ33 238700277.

0191-8141/$ e see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jsg.2011.03.003

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a b s t r a c t

Regional strike-slip faulting can control magma movements, deform volcanoes and may destabilise theirflanks. The aim of this study is to address this problem by comparing two natural examples, Basse TerreIsland volcanoes, Lesser Antilles and Maderas volcano, Nicaragua, with analogue experiments. The fieldand remote sensing analyses of their structures reveal that Guadeloupe volcanoes, which developed ina 145�-striking sinistral transtensional fault zone, are dominantly fractured in a 090�e120� direction,which is parallel to the maximum principal horizontal stress and to the elongation direction of thesummit graben of analogue models. This graben is bordered by the Sigmoid-I fault, or Y shear structure,and has facilitated the formation of the Beaugendre and Vieux-Habitants valleys by faulting, erosion orcollapse. This structure has also influenced the injection of dykes and the transport of hydrothermalfluids. The comparison of Maderas volcano with the analogue models confirms that the volcano hasdeveloped parallel to a 135�-striking dextral transtensional fault zone and is also gravitationallyspreading over a weak substratum. This study illustrates how regional strike-slip faulting and gravita-tional loading combine to produce a clear set of structures within volcanic edifices, which control thelocation of intrusive zones, hydrothermal activity and collapse directions.

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1. Introduction

The constructional morphology of a volcanic edifice can bemodified by regional faulting and by local processes such as grav-itational spreading (Dusquenoy et al., 1994; Bourne et al., 1998;Groppelli and Tibaldi, 1999; Corpuz et al., 2004). This paperexamines the interaction between regional strike-slip faults andstable or spreading conical edifices. The structure of Basse TerreIsland volcanoes, Guadeloupe, Lesser Antilles, and Maderasvolcano, Ometepe Island, Nicaragua, are investigated by field andremote sensing studies and interpreted with analogue models.

The analogue models that have been conduced to grasp theinteraction between volcanic edifices and strike-slip, transpres-sional and transtensional faults are described in the part 1 of thisdouble-paper. Themodels consist of a cone of granularmaterial anda brittle or partially ductile substratum, which are sheared bystrike-slip, transtensional or transpressional faults located beneath

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the cone summit or located a few centimetres from it for a set weterm offset experiments. The models indicate that sheared conesdevelop a summit graben and two curved Sigmoid-I and II faults,which develop 10�e20� from the regional fault as described byLagmay et al. (2000).

Sigmoid-I is a major synthetic fault, which corresponds to the Yshear structure (cf. Sylvester, 1988). At the summit of the volcano,the Sigmoid-I fault has a transtensional motion and bordersa summit graben elongated in a direction parallel to the mainhorizontal contraction, or sigma 1 stress. The Sigmoid-II faultaccommodates more movement as the extensional component ofthe regional fault increases and is thus well developed only fortranstensional experiments. The addition of a ductile substratumincreases the extensional component accommodated by theSigmoid-I and II faults and forms 2 broad shallow summit grabensparallel to the main horizontal stress and to the regional fault zone.This kinematic description of the models is used to interpret thestructure of the Guadeloupe and Maderas volcanoes and to char-acterise the movement of magma in these volcanoes.

The displacement maps that have been established from theanalogue models describe the direction and amplitude of hori-zontal movement of the cone flanks throughout an experiment(Mathieu, 2010). In brittle substratum experiments, the fault zone

en volcanoes and strike-slip, transtensional and transpressional faultlogy (2011), doi:10.1016/j.jsg.2011.03.003

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L. Mathieu et al. / Journal of Structural Geology xxx (2011) 1e92

bordered by the Sigmoid-I and II faults encloses the fastest movingand most unstable upper cone flanks and summit (Lagmay et al.,2000; Andrade, 2009). In ductile experiments, the fastest move-ments are located at the periphery of the fault zone, overa restricted area of the cone’s lower flanks (Mathieu, 2010). Theseresults are used to predict the likely location of collapse events onvolcanic cones.

The theoretical considerations on dyke and collapse locationsdeduced from analogue models are compared with Basse TerreIsland and Maderas volcanoes. Basse Terre Island volcanoes arestudied because of their dominantly brittle basement, whichcontains an active sinistral transtensional fault zonewell imaged bybathymetric studies (Feuillet, 2000; Fig. 1a). Maderas is studiedbecause its small size makes it easy to investigate in the field andbecause of the structure of its basement, which contains an activetranstensional fault zone and a ductile layer (DeMets, 2001; Borgiaand van Wyk de Vries, 2003; Fig. 1b).

2. Natural examples

The two volcanoes presented in this paper are covered witha tropical rain forest and the field study was mostly carried out inriver beds, which provide scattered outcrops. Remote sensing data,in particular the Digital Elevation Models (DEM), complete the fieldanalysis.

2.1. Basse Terre Island, Guadeloupe Archipelago, Lesser Antilles

2.1.1. Presentation of the volcanoesThe Basse Terre Island is made of late Tertiary to Quaternary

volcanoes, which form the western part of the Guadeloupe Archi-pelago, Lesser Antilles volcanic arc (Fig. 1a). It is a 50 km long,1467 m high island built on the thick Caribbean oceanic crust andformed by the oblique subduction of the Atlantic oceanic crust. TheNE part of the Caribbean plate is internally deformed by obliquesubduction (Feuillet, 2000; Fig. 1a). In the vicinity of Basse TerreIsland, this deformation formed the 140�e160�-strikingMontserrat-Bouillante sinistral transtensional fault, which inter-sects Basse Terre in the vicinity of the Bouillante town (Feuillet,2000; Thinon et al., 2010). Structures parallel to this fault are

Fig. 1. a) Map of the Lesser Antilles volcanic Arc. The regional tectonic framework is from FeuWeber et al. (2001); b) Regional setting of Nicaragua (after Cailleau et al. 2007; DeMets, 20

Please cite this article in press as: Mathieu, L., et al., The interaction betwezones: Analogue models and natural examples, Journal of Structural Geo

observed on bathymetry data and in outcrops in the Grande TerreIsland (Feuillet, 2000) and they define the 50 kmwide Monsterrat-Bouillante fault zone (Fig. 2b), which encompasses Basse TerreIsland volcanoes as well as the E-W-striking Marie-Gallante graben(Fig. 2b). The Basse Terre Island is an assemblage of compositevolcanoes, which are, from north to south: 1) the Basal Complex(w2.7 Ma); 2) the Northern Chain (1.8e1.1 Ma); 3) the Axial Chain(1e0.6 Ma); 4) the Grande Découverte volcano and other recentedifices (0.2 Ma-recent) and 5) the Mt Caraïbe (0.5 Ma; age datafrom Samper, 2007; Fig. 2a).

2.1.2. Results of field analysisThe authors have conducted an extensive field study of Basse

Terre Island, which is available inMathieu (2010). The description ofthe island presented hereafter is based on the result of this analysis.

Field and remote sensing analyses describe the progressiveformation of the Basse Terre Island through successive eruptionsand collapse events, enlarging on the work of Boudon et al. (1992)and Komorowski et al. (2005). This interpretation considers theBasal Complex and the Northern Chain (Fig. 2a) to have been builton 160�-striking structures (Feuillet, 2000; Samper, 2007) parallelto the subduction front (Mathieu, 2010). Older, contemporaneous(cf. the 3.5 Ma and 2.5 Ma Directeur and Vieux-Fort seamounts;Bouysse et al., 1985), and younger volcanoes such as the Axial Chainand Grande Découverte volcanoes, have been emplaced parallel tothe Montserrat-Bouillante transtensional fault zone, along whichthemagma is likely to have been transported (Feuillet, 2000; Fig. 2).

The Piton Bouillante volcano (Fig. 2a; Mathieu, 2010), whichforms the northern part of the Axial Chain, was emplaced south ofthe Northern Chain and dissected by the large valleys of Beau-gendre and Vieux-Habitants, which originated either from calderacollapses (Westercamp and Tazieff, 1980; Dagain, 1981), sectorcollapses (Boudon, 1987; Feuillet, 2000; Samper, 2007; Mathieu,2010) or from erosion (Samper, 2007; Fig. 2). The lack of thickpyroclastic deposits, felsic dyke injections and dome extrusionsenables us to reject the caldera hypothesis. The sector collapsetheory could not be confirmed by field observations, mostlybecause any debris avalanche deposits produced by such eventswould have been eroded by the sub-marine valleys located west ofthe collapse scars. However, we favour a sector collapse origin to

illet (2000) and the plate motion is from Bouysse et al. (1990), DeMets et al. (2000) and01, van Wyk de Vries, 1993).


Fig. 2. a) Map summarizing the main structures of Basse Terre Island; b) Regional setting of Basse Terre Island, Guadeloupe Archipelago (modified after Baubron, 1990; Feuillet,2000; Thinon et al., 2010).

L. Mathieu et al. / Journal of Structural Geology xxx (2011) 1e9 3

account for the formation of the two abnormally large and deepBeaugendre and Vieux-Habitants valleys.

About 0.5 Ma ago, intense volcanic activity formed threevolcanic edifices inside the Vieux-Habitants valley (Samper, 2007),as well as the Les Mamelles domes north of Piton Bouillantevolcano (Mathieu, 2010) and Mt Caraïbe at the southern extremityof the island (Bouysse et al., 1985; Blanc, 1983; Fig. 2a). Thesevolcanoes are also aligned along a segment of the 140�-strikingMontserrat-Bouillante fault zone (Mathieu, 2010).

The fractures, veins and faults attributed to regional stresses byour field study strike 160�e000�, 140� and 090�e120�. The 140�-striking fractures and faults parallel to the Montserrat-Bouillantefault zone are little represented by field data (Mathieu, 2010). Theabundant 160�e000�-striking fractures and the rare parallel faults,


including the 170�-striking Ty fault (La Soufriere dome area; Julienand Bonneton, 1984), have been observed throughout Basse TerreIsland (Fig. 2a). These structures, as well as seven 160�e000�-striking dykes, are likely related to fracturing of the crust parallel tothe subduction front during uplift or to simple lava emplacementand erosion over N-S-striking slopes (Mathieu, 2010).

The090�e120�-striking structuresarebetter represented than the160�e000� and 140� trends and correspond to the 120�-strikingCapesterre fault (Baubron, 1990), to the E-W-striking normal faultsdrilled in the Bouillante towngeothermalfield (cf. Fig. 2a for location;Traineau et al., 1997; Lachassagne et al., 2009) and to the many090�e120�-striking fractures, faults and 55 dykes observed else-where in the island (Mathieu, 2010). Several E-W-striking normalfaults have also been observed (Feuillet, 2000) and measured



(Mathieu, 2010) along the western shore of Basse Terre Island.However, these structures are located in loose pyroclastic and debrisflow deposits and may have accommodated the deposition anderosion of these rocks rather than any regional faultmovements. The090�e120�-striking structures are parallel and likely related to themaximum regional horizontal stress, which strikes 090� � 2�

(DeMets et al., 2000;Weber et al., 2001),102� (Bouysse et al.,1990) or120� � 10� (Chabellard et al., 1986; Julien and Bonneton, 1984;Heidbach et al., 2008).

2.2. Maderas volcano, Ometepe Island, Nicaragua

2.2.1. Presentation of the volcanoMaderas is a stratovolcano which, along with Concepcion

volcano, makes up the Ometepe Island in Lake Nicaragua (Fig. 1b).This small volcano is 1345 m high, has a diameter of 11 km and hasnot erupted for at least 3000 years (van Wyk de Vries, 1993).Maderas volcano is a case-study of a spreading volcano, whichspread over weak lake sediments (van Wyk de Vries, 1993; Borgiaand van Wyk de Vries, 2003; Borgia et al., 2000) and which haspossibly developed in an E-W directed extensional stress field anddiffuse regional dextral strike-slip shear hidden by Lake Nicaraguasediments (van Wyk de Vries and Merle, 1998).

Maderas volcano is one of the 21 Nicaraguan arc volcanoes of theQuaternary Central American volcanic front. These small edificeshave individual volumes of about 30 km3 and are related to theoblique subduction of the Cocos oceanic lithosphere beneath thecontinental or ophiolithic Caribbean crust (vanWyk de Vries, 1993;Fig. 1b). The 10� obliquity of the subduction (DeMets, 2001) is likelyresponsible for the rotation of the northernmargin of the Caribbeanplate and for the formation of regional crust faults and neo-struc-tures, which strike NE (sinistral strike-slip faults; La Femina et al.,2002), N-S (transtensional faults) and NW (normal faults; Cruden,1991; Manton, 1987; van Wyk de Vries, 1993). In a regional contextof oblique subduction, and by analogy with the Lesser Antillessubduction, we propose that dextral strike-slip fault zones mayoverprint the NW-striking volcanic front. Similar faults are clearlyseen elsewhere in Nicaragua (cf. the Masaya-Mateare fault zone;Girard and van Wyk de Vries, 2005) and, for example, in Salvador(Corti et al., 2005; Alvarado et al., 2011). It is proposed to test thishypothesis by comparing the structure and morphology of theMaderas volcano with the structures observed in analogue models.

Fig. 3. a) Structural map of Maderas volcano which combines several remote sensing observaflank vents (scoria cones) are not visible on remote sensing data and are only documented bazimuth ¼ 045).


2.2.2. Remote sensing observationsThe structure of Maderas volcano is studiedwith remote sensing

observations of: 1) SRTM (Shuttle Radar Topographic Mission) 3 arcseconds (e.g. square pixels are 90 m over 90 m) DEMs; 2) aerialphotographs and 3) field observations of the volcano morphologymade from the base of the edifice. The SRTM images and the fieldobservations indicate that Maderas volcano has a 135�-strikingsummit graben and steepNE-E andW-SWupperflanks (Fig. 3a). Theaerial photographs provide more details on the location andmorphology of the 135�-striking structures and on the vents locatedat theN-NEbaseof the edifice. In order tomakeamorecomplete anddetailed structuralmap of the volcano, the authors also digitized thecontour levels (interval¼ 20m) of a topographicmap of the volcanoand transformed the data into a 1 arc second (resolution ¼ 30 m)DEM using ENVI software (e.g. DEM_30 m; Fig. 3b). The lineamentsinterpreted from this document are compared with those obtainedfrom the previously described sources and are presented on Fig. 3a.

The 135�-striking graben is a major structure and is intersectedby a 000�-striking lineaments. Near-radial lineaments are observedon the lower flanks of the volcano. These lineaments are organisedin pairs of escarpments facing each other and intersecting the baseof the edifice (Fig. 3a).

2.2.3. Field dataMaderas volcano is covered with dense rain forest; it is small in

size and dormant. These three characteristics do not favour goodquality outcrops and they are indeed rare, of limited extent andhighly weathered, mostly because the long dry season prevents thedevelopment of many permanent streams which usually providegood quality outcrops. Evidence of explosive eruptions is indicatedby the presence of surge and fallout deposits on top of a lava pileobserved along Mérida River, west of the summit. These depositsmay have originated either fromMaderas or from the neighbouringConcepción volcano. Two eroded vents were identified on thevolcano’s upper flank, west and north of the summit crater (Fig. 3a).The thickest lava flows are found on the SSE flank of the volcano(e.g. Tichana River), between two 135�-striking cliffs.

2.2.4. The structural map of Maderas volcanoMaderas possesses a flat summit bordered by 135�-striking

faults (van Wyk de Vries and Merle, 1998) and its steepest upperflanks are located east and west of its summit (Fig. 3a). The lower

tions (e.g. aerial photography, SRTM, DEM_30 m and field observations). The two uppery field observations; b) DEM_30 m presented as a hill shade map (sun elevation ¼ 45� ,



flank pairs of escarpments are half-grabens similar to the flowerstructures that develop on volcanic and analogue model conesaffected by gravitational spreading (e.g. Merle and Borgia, 1996;Delcamp et al., 2008; van Wyk de Vries and Matela, 1998). Thehalf-grabens accommodate the stretching of the lower flanks that isinduced by the spreading of the volcanic cone. The flat summit andclear grabens indicate that the spreading is affecting a dormantvolcanic edifice. If Maderas had a vigorous activity, summit erup-tions would build a steep summit cone faster than the spreadingmovements could flatten it. When the volcano was active, thespreading movements favoured summit extension, the develop-ment of a graben and the establishment of a central conduit, whichis evidenced by the presence of a summit crater.

The most recent eruptive vents have a preserved topographyand are located at the N-NE base of the volcano (Fig. 3a). They mayhave been fed bymagma accumulated inweak horizons such as theLake Nicaragua sediments or the thrust and strike-slip faults, whichaccommodate the spreading movements at the base of the volcano.The vents are hydrovolcanic at this lower elevation due to theinteraction between the magma and the Lake Nicaragua. Similarvents associatedwith faults are seen on Concepción volcano (Borgiaand van Wyk de Vries, 2003).

Several eroded vents have developed in the 135�-strikingstructures and the thickest lava flows (SE flank) may have beenconfined along these structures. These two observations indicatethat the 135�-striking faults formed before the end of the volcanicactivity. The faults have freshmorphology (e.g. steep cliffs) andmaythus still be active structures.

The regional tectonic setting suggests that Maderas volcano hasdeveloped in a 135�-striking dextral transtensional fault zone, asalready suggested by vanWyk de Vries andMerle (1998). The 135�-striking graben is likely to be parallel to the regional fault zone. Thesummit crater is located on top of this fault zone that facilitated thetransport of magma in the crust (Fig. 3a). According to thishypothesis the 000� lineaments would correspond to the fault zonetension structures emphasised by the spreading, which tends tofavour extensional movements. This hypothesis will be confrontedto the analogue experiment results.

3. Implications of analogue models for natural examples

3.1. Implications for the transport of magma

We have imposed a regional deformation and stress field onanalogue cones and we have described the structures that havedeveloped in cones underlain by a regional strike-slip fault (cf. part1 article). For technical reasons, as repeated dyke intrusion in brittlematerial is impracticable, our models do not take into account themovement of magma in the analogue cone and a theoreticalapproach is favoured to discuss this fundamental characteristic ofvolcanic edifices.

It has long been recognised that dykes and vent alignments areparallel to the greatest principal stress (Nakamura, 1977). In ourmodels, the main horizontal stress is parallel to the elongationdirection of the summit graben, to which most dyke injections arelikely follow in nature (Fig. 4). This hypothesis has been confirmedby single intrusion analogue models by Andrade (2009). Accordingto van Wyk de Vries and Merle (1998), the cone summit graben isthe result of the cone loading effect on the regional fault and, byfavouring more magma influx, produces more load and promotesfurther summit extension. This feedback relationship can promotesummit injections and eruptions may develop.

In transtensional experiments, the summit graben is encom-passed by a deeper and longer graben, which is bordered bySigmoid-I and II faults andwhich forms parallel to the Yplane of the


regional fault zone (Fig. 4). In the case of low viscosity magmainjection, themagma is expected to rise along the regional fault zoneand to form a dyke swarm, or volcanic rift zone, parallel to the deepgraben in the cone. In the case of a magma too viscous to besystematically injectedasdykes, an alignmentof volcanic edifices, ordomes, is expected to form parallel to the regional fault plane. Instrike-slip experiments, Sigmoid-II faults border a subsiding upperflank to summit area to which eruptions are expected to berestricted. Eruptions may be restricted to the Sigmoid-I summitgraben over regional transpressional faults. Finally, the bulk of conefaults are susceptible to be infiltrated by minor magma injectionsoblique to the main stress axes.

The addition of a ductile substratum increases the extensionalcomponent of the faults. In such a context, a greater volume ofmagma, with lower buoyancy, may be injected along the conefaults. The dyke injections may push the flanks in a directionperpendicular to the summit graben elongation direction,increasing the amplitude of spreading movements of these pushedflanks, and favour further dyke injections (Fig. 4). The cone flankslocated on each side of the summit graben may be thus be pushedand they correspond to the fastest moving flanks of the experi-mental cones according to the displacement maps (Mathieu, 2010).

3.2. Defining unstable flanks

The analogue cones sheared by pure strike-slip faults describedin the literature have systematically been used to determine thelocation of unstable cone flanks that are the most likely to beaffected by large avalanches or other smaller volume collapseevents. It has thus been determined that collapse events are likelyto develop in a volcano within 10�e20� from a regional pure strike-slip fault plane (Lagmay et al., 2000; Norini and Lagmay, 2005;Wooller et al., 2009) and to be restricted to the fault zone definedby Sigmoid-I and II faults in the vicinity of a regional transtensionalfault (Norini et al., 2008).

Catastrophic collapse eventsmayhavemanyorigins, one of thesebeing thedykedilatation. Flank failuremay thus occur parallel to themaximum horizontal stress (Moriya, 1980), in a direction perpen-dicular to the summit graben elongation direction. Flank failuremayalso affect the flanks located on each side of the volcanic rift zonethat may be hosted by the fault zone defined by Sigmoid-I and IIfaults, in a regional transtensional context (cf. Delcamp et al., 2010).

The regional strike-slip movement may also have a directimpact on the collapse events by shearing and displacing thevolcano flanks. The Sigmoid-I fault crosses the whole cone frombottom to top and from one side to the other, while Sigmoid-II isabsent or restricted to the summit area. Thus, the major fault andmost likely discontinuity to be affected by a collapse scar isSigmoid-I. Additionally, the displacement maps of ductilesubstratum experiments indicate that the lower cone flanks locatedon each side of the summit graben are affected by the fastesthorizontal movements. These flanks move away from the cone’ssummit and are in extension. On the other hand, folds are observedat the base of the flanks with a slope direction parallel to sigma 1and these flanks are in compression (Fig. 4). We propose that theflanks that are in compression are likely to be internally deformedand may be affected by superficial collapses. However, a largecollapse event is more likely to be bordered by Sigmoid-I fault andto affect the flanks which are in extension. According to thishypothesis, the largest collapse events may be located on each sideof the summit graben and be directed in a direction normal to themain horizontal stress (Fig. 4).

In summary, the collapse direction proposed by Lagmay et al.(2000), Norini and Lagmay (2005) and Wooller et al. (2009) arerestricted to the fault zone defined by Sigmoid-I and II faults and


Fig. 4. Sketches summarizing the likely location of dykes and collapse scars in cones that develop in the vicinity of sinistral strike-slip, transtensional and transpressional faultzones.


occur in the direction of the thrust movements accommodated bySigmoid-I fault. We propose an additional collapse direction, whichis normal to sigma 1, to the strike of the dyke injections and affectsthe cone flanks that are in extension. The two models are notincompatible with each other and may both explain the occurrenceof successive collapse events of different volumes along most of theflanks of a volcano.

The analogue cones behave differently when there is an offsetbetween the fault zone and the cone summit (cf. Offset experi-ments; Fig. 4). In this case, the small cone flank (part B; Fig. 4) isextruded and slides along the well developed part A-Sigmoid-IIfault. This sliding is analogue to a sector collapse in nature. It affectsexclusively the half-part B flank which is bordered by Sigmoid-IIfault in our models (Fig. 4).

3.3. Implication for Guadeloupe volcanoes

This section focuses on the southern volcanoes of the BasseTerre Island, on the Axial Chain and Grande Découverte volcanoes,which sit on the 145�-striking sinistral transtensional Montserrat-Bouillante fault (Fig. 5a). The volcanoes do not possess a clear


regional fault parallel deep graben observed in experimental cones,but such a structure may be hidden by magma output. The magmaof Basse Terre Island are also possibly too viscous to enable theformation of a well defined volcanic rift zone and, instead, thecomposite volcanoes are aligned parallel to the regional fault zone.

The volcanoes possess abundant 090�e120�-striking fractures,faults and dykes (Fig. 5a). Guadeloupe volcanoes are also charac-terised by repeated sector collapses (e.g. Komorowski et al., 2005).Field data are abundant on the Piton Bouillante volcano, which isa circular edifice with many 090�e120�-striking exposed dykes andthe two large valleys of Beaugendre and Vieux-Habitants, whichwere formed by sector collapses and/or erosion (Fig. 5a). The090�e120�-striking dykes are parallel to the main horizontal stressand develop 55�e25� from the regional fault. This orientation is thesame as the maximum principle horizontal stress of the transten-sional experiments (Fig. 5b). The variability on the dyke orientationdid not enable us to distinguish between ductile and brittlesubstratum experiments. However, there is no clear evidence toindicate that Guadeloupe volcanoes are undergoing significantwhole-scale spreading and it is reasonable to assume that they arecomparable to brittle substratum experiments.


Fig. 5. a) Structural sketch of the southern part of Guadeloupe Island, Lesser Antilles; b) Picture of experiment C33 (sinistral transtensional fault, a ¼ 20� , brittle substratum).


Note that the Guadeloupean dykes are not parallel to theregional fault zone, as would have been expected from transten-sional experiments (see previous section) because they areobserved in the Piton Bouillante volcano upper flank area (Fig. 5a).There, they are parallel to the equivalent model summit graben andto the maximum principal horizontal stress. Based on the experi-ments, buried dykes located beneath the lower flanks of PitonBouillante, if present, are predicted to strike 130� and 160�, parallelto Sigmoid-II and I, respectively.

The possible collapse scars (Beaugendre and Vieux-Habitantsvalleys) cut into the southern, SWandwestern flanks of the volcano(Fig. 5a). By comparison with the analogue experiments, theunstable area is delimited by Sigmoid-I fault (S-SSE of PitonBouillante peak; e.g. Fig. 2 for location), by the summit graben onthe north (Piton Bouillante peak area) and it comprises the SW coneflank, which would be in extension according to the models (cf.Fig. 4). The avalanche orientation is normal to the summit grabenelongation and corresponds to the large-scale collapse by analoguemodels. Alternatively, these two valleysmay also originate from theerosion of flanks heavily fractured by the Sigmoid-I structure. Thevalleys are located on the only flank which was not buttressed byother volcanoes and was, at the time of the possible avalancheevents, facing the sea (e.g. Le Friant, 2001). The regional faultmovement is a factor among others, which may have destabilisedthe SW flank of Piton Bouillante volcano.

Finally, the Sigmoid-I fault is a steeply dipping structure thatcuts through the heart of the edifice to its base and is the mostlikely to connect with the hypovolcanic complex and to channelhydrothermal fluids, as has been observed on other volcanoes(Lagmay et al., 2003). The fluids of the Bouillante geothermalsystem (Fig. 5a) may thus have been transported by Sigmoid-I fault,which strikes 160� in the flank area, where the geothermal field isobserved at the surface, to about E-W in the summit area, wherehidden hot fluids may circulate.


Note that, as Grande Découverte volcano was building south ofPiton Bouillante volcano, the Sigmoid-I fault has likely been shiftedtowards the south to encompass the summit of this youngervolcano (Fig. 5a). The present-day area which is in extension andsusceptible to be affected by collapses, corresponds to the SW partof Grande Découverte volcano. This area experienced two recentavalanches, 3100 and 11,500 years ago (Boudon, 1987).

3.4. Implication for Maderas volcano

Maderas is a small and dormant volcanic cone with a circularbase. It sits on a 135�-striking strike-slip or transtensional fault andit spreads over its weak substratum. It possesses a 135�-elongatedgraben, which has developed parallel to the regional fault zonetrend (Fig. 6a). This graben corresponds to the subsiding structurebordered by Sigmoid-I and II faults of transtensional experiments(Fig. 6b). This extensional structure facilitates the rise of magmatowards the surface as evidenced by the 5 vents and the summitcrater that are located along it. Maderas volcano also possessesseveral 000�-striking lineaments whose kinematics could not bedetermined in the field. These structures correspond to the models’shallow grabens that form 45� from the regional fault plane intranstensional and ductile substratum experiments (Fig. 5b). The000�-striking lineaments of Maderas form also 45� from theregional fault and are thus, according to the models, likely to benormal faults. The location of these structures (e.g. rotated clock-wise from the regional fault) indicates that the regional fault hasa dextral sense of motion (Fig. 5a).

The analogue experiments indicate that Maderas volcano wasbuilt on a transtensional fault zone with a right-lateral motion. Thepercentage of the extensional over the compressional componentof movement of this fault could be approximated if these compo-nents were known over the 135�-striking transtensional faults ofMaderas volcano. This information cannot be obtained from the


Fig. 6. a) Structural sketch of Maderas volcano, Nicaragua. Note that the exact location and throw direction of the regional fault zone are unknown; b) Picture of experiments C19(dextral transtensional fault, a ¼ 20� , brittle and ductile substratum).


rare and weathered outcrops of the area but may be investigatedwith geodetic-GPS studies.

4. Conclusion

Basse Terre Island of the Guadeloupe Archipelago is an assem-blage of composite volcanoes, which developed in an active sinis-tral transtensional fault zone. The Sigmoid-I structure strikes about140�e160� and curves to 090�e120� in the upper flank area of theabout 1 Ma old (Samper, 2007) Piton Bouillante volcano and hasmore recently extended at the summit of the younger GrandeDécouverte volcano. Sigmoid-I favoured the injection of090�e120�-striking dykes and the circulation of hydrothermalfluids along 090�e120�-striking faults and fractures, creating theBouillante geothermal field. The regional fault movement has alsoinfluenced the formation of two large erosion and/or collapse scarvalleys on the Piton Bouillante volcano (e.g. Beaugendre and Vieux-Habitants valleys) and recent debris avalanches on the GrandeDécouverte volcano (e.g. 3100 and 11,500 BP events).

Maderas volcano was built on top of a 135�-striking dextraltranstensional fault zone and has spread over its ductile substratum.It possesses a 135�-striking graben bordered by Sigmoid-I and IIfaults, which has favoured the rise of magma. The regional (trans-tensional fault) and local (spreading) stress fields have favoured theestablishment of a central conduit (e.g. summit crater). The gravi-tational spreadinghasflattened the summitof this dormantvolcano,has formed half-grabens in the volcano lower flanks and may havefavoured the formation of vents at the base of the volcano.

Analoguemodelsareused to identify thepreferential area fordykeinjections (summit graben), the fracture zones likely to transport thehydrothermalfluid (Sigmoid-I fault) and the unstableflanks (NWandSE flanks) of Piton Bouillante (Guadeloupe) and Maderas volcanoes.Thesemodelsmaybeused tobetterunderstand the structureofmanyother volcanoes, which have developed in the vicinity of faults thatpossesses a strike-slip component of movement.

Acknowledgements

The authors wish to thanks Nelly Mazzoni, Claire Mannessiez,Elodie Lebas, Cécile Savry and Lara Kapelanczyk for their assistancein the field. The PhD of L. Mathieu was funded by IRCSET (Irish


Research Council for Sciences, Engineering and Technology). TheGuadeloupe fieldwork was supported by the ANR-VOLCARISK.

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